The present invention relates to a voltage output circuit of IC (integrated circuit) structure.
A typical example of the conventional voltage output circuit of IC structure is illustrated in FIG. 1. This circuit is described in "IEEE JOURNAL OF SOLID-STATE CIRCUITS, VOL, SC-6, No. 1 FEBRUARY 1971, PP 1 to 7". The conventional voltage output circuit was developed in use for an integrated one operated at a low voltage.
A voltage (V.sub.CC) from a power source is applied to a constant current source 11. The output current of constant current source 11 is fed through a resistor R1 to the collector of an NPN transistor Q1. Transistor Q1 and another NPN transistor Q2 are designed to have different current densities. Specifically, a current density of transistor Q2 is approximately 1/10 that of transistor Q1. In this circuit, a voltage drop .DELTA.VBE, which has a positive temperature coefficient, appears across a resistor R3 connected between the emitter of transistor Q2 and the ground providing a reference potential. Further, a voltage drop .DELTA.VBE.times.R2/R3, which has a positive temperature coefficient, appears across a resistor R2 connected between the collector of transistor Q2 and a voltage output portion 12. In the above mathematical expression, R2 and R3 designate resistors having a resistance denoted by the same reference characters R2 and R3. In this circuit, a voltage VBE3 across the base-emitter path of an output NPN transistor Q3 has a negative temperature coefficient. By such a selection of the temperature coefficients, the temperature coefficient of the reference output voltage Vref is zeroed.
Generally, the output NPN transistors of the circuits have unequal current amplification factors .beta. on the technology for manufacturing the circuits. Therefore, output voltages of the circuits are unequal. When the factor .beta. is small and a large base current flows, a large voltage drop appears across resistor R2 and a large output voltage Vref appears. On the other hand, when the factor .beta. is large and a small base current flows, a small voltage drop appears across resistor R2 and a small output voltage Vref appears. Resistor R2 is made of a base diffusion resistor which has no relation with the current amplification factor .beta. of the NPN transistor.